Voltage clamp

ABSTRACT

Voltage clamping is provided. A first reverse direction high-electron-mobility transistor includes a source and a gate connected to a voltage clamped line, and a drain connected to a first reference voltage. A second reverse direction high-electron-mobility transistor includes a source and a gate connected to a second reference voltage, and a drain connected to the voltage clamped line.

BACKGROUND

A metal-oxide-semiconductor field-effect transistor (MOSFET) uses aninsulated gate to control current flow between a source and a drain ofthe MOSFET. Current Voltage characteristics of a conventional MOSFET areshown in FIG. 1. In FIG. 1, the horizontal axis represents voltage fromthe drain to the source (Vds). The vertical axis represents currentvalues flow from the drain to the source (Ids). As long as the MOSFET isforward biased (Vds is positive), the gate-to-source voltage(Vgs)—sometimes called gate voltage Vg—controls current flow (Ids)through the MOSFET. The threshold voltage (Vth) is the minimum value ofVgs that is needed to create a conducting path between the source andthe drain. As illustrated in FIG. 1, increasing the gate voltage abovethe threshold voltage results in increased conductivity.

When the MOSFET is negative biased (Vds is negative), the gate-to-sourcevoltage (Vg) has less impact on current flow through the MOSFET. This isthe result of a body diode intrinsic within FETs which allows currentflow from source to drain regardless of the gate voltage. For example,in an n-channel MOSFET, the source and the drain are n+ regions and thebody is a p region. The p-n junction formed at the intersection of the pbody and the n+ regions act as a diode between the body and the sourceof the MOSFET and between the body and the drain of the MOSFET. Becausein a MOSFET the source is typically shorted to the body, the body diodebetween the body and the source is irrelevant. However, the body diodeto the drain allows a current path from the body to the drain when theMOSFET is negative biased (Vds is negative).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows current characteristics of a typicalmetal-oxide-semiconductor field-effect transistor (MOSFET) in accordancewith the prior art.

FIG. 2 shows current characteristics of a high-electron-mobilitytransistor (HEMT).

FIG. 3 is a simplified circuit diagram of a voltage clamping circuit.

FIG. 4 is a simplified circuit diagram showing a voltage clampingcircuit providing electrostatic discharge protection for an input pad ofan integrated circuit.

DETAILED DESCRIPTION

A high-electron-mobility transistor (HEMT) also known as aheterostructure FET (HFET) is a field-effect transistor incorporating ajunction between two materials with different band gaps at the channelinstead of a doped region. In a Gallium Arsenide (GaAs) HEMT, a depletedAluminum Gallium Arsenide (AlGaAs) layer is placed over a non-dopednarrow-bandgap channel layer of GaAs. The electrons generated in thethin n-type AlGaAs layer drop into the GaAs layer to form a depletedAlGaAs layer. The heterojunction created by different band-gap materialsforms a quantum well in the conduction band on the GaAs side where theelectrons can move quickly without colliding with any impurities. Thiscreates a very thin layer of highly mobile conducting electrons withvery high concentration, giving the channel very low resistivity. Othermaterials can be used to form a HEMT such as in a Gallium Nitride HEMT.GaN-based HEMTs have a similar layered structure where no intentionaldoping is required. In AlGaN/GaN HEMTs, electrons form a high carrierconcentration at the interface, which leads to a two-dimensionalelectron gas (2DEG) channel due to the spontaneous polarization found inwurtzite-structured GaN. The 2DEG is a function of AlGaN thickness andthe bound positive charge at the interface. AlGaN/GaN HEMTs providinghigh power density and breakdown voltage can be achieved. Thepolarization effect between the GaN channel layer and AlGaN barrierlayer causes a sheet of uncompensated charge in the order of 0.01-0.03Coulombs per meter (C/m) to form. If the 2DEG is continuous betweensource and drain the transistor will be normally on or depletion HEMT(dHEMT) turning off with a negative gate bias. With the addition of Mgdoping or other techniques to compensate the built in charge under thegate, the 2DEG is not continuous at zero gate bias. This will achieve anormally off or enhancement mode behavior characteristic of anenhancement HEMT (eHEMT).

Additional eHEMT devices of interest are Indium Phosphate (InP) basedHEMTs due to their high electron mobility, high electron saturationvelocity, and high electron concentration. These devices are made of anInGaAs/InAlAs composite cap layer, an undoped InAlAs Schottky barrierand an InGaAs/InAs composite channel for superior electron transportproperties.

Since there are no p-n junction within an HEMT, there is no p-n bodydiode formed. This results in significantly different voltagecharacteristics between a HEMT and a MOSFET. For example, FIG. 2 showscurrent voltage characteristics of a HEMT. In FIG. 2, the horizontalaxis represents voltage from the drain to the source (Vds). The verticalaxis represents current values flow from the drain to the source (Ids).HEMT transistor current-voltage characteristics in the forward directionlook similar to PN junction technologies like MOSFETs. That is, as longas the HEMT is forward biased (Vds is positive), the gate-to-sourcevoltage (Vgs) controls current flow (Ids) through the HEMT.

The reverse conduction characteristics of a reverse direction HEMT(RDHEMT) are different than the reverse conduction characteristics ofMOSFETS because in HEMTs there is no p-n body diode formed. In additionto the ability to block reverse voltages above the typical 0.6 volts offorward biased silicon PN junctions, some HEMT transistors turn on inthe reverse direction with a negative voltage on the drain relative tothe source (−Vds) primarily due to charge injection into the enhancementmode channel. This category of HEMT transistors have reverse conductioncharacteristics that differ from their forward conductioncharacteristics in both cause and form.

For example, Gallium nitride HEMTs are an example of HEMT transistorsthat have a reverse conduction mode and have attracted attention due totheir high-power and high frequency performance. In the reversedirection, such an RDHEMT device starts to conduct when the absolutevalue of the negative drain voltage with respect to the source voltage|−Vds | is greater than the gate threshold voltage. The RDHEMT thenexhibits a channel resistance and conducts current. If a negative gatevoltage is applied with respect to the source voltage, the negativedrain to source voltage must be increased for the RDHEMT to conductcurrent.

FIG. 3 is a simplified circuit diagram of a voltage clamping circuit 109used to clamp voltage excursions by using RDHEMT operation in thereverse direction.

An RDHEMT 100 has a source 101, a drain 102 and a gate 103. An RDHEMT110 has a source 111, a drain 112 and a gate 113. Source 111 and gate113 of RDHEMT 110 are connected to a reference voltage 106 (−V). Drain102 of RDHEMT 100 is connected to a reference voltage 105 (+V). Source101 and gate 103 of RDHEMT 100 and drain 112 of RDHEMT 110 are allconnected to a line 107 that is voltage clamped.

Because source 101 and gate 103 of RDHEMT 100 are connected to line 107,line 107 is voltage clamped from being significantly more positive thanreference voltage reference voltage +V. When the voltage on line 107 isincreased to be much greater than reference voltage +V, the drain tosource voltage or Vds of RDHEMT 100 will decrease and go negative. Asthe voltage on line 107 continues to increase, the magnitude of thenegative drain to source voltage of RDHEMT 100 will continue to increaseuntil RDHEMT 100 begins to conduct current in the reverse direction fromline 107 through to reference voltage 105 (+V), resulting in a voltageclamping effect on line 107.

The operating characteristics of RDHEMT 100 are illustrated in FIG. 2 asseen for the case where Vgs=0. When Vgs=0 and Vds is greater than −1.6volts, there is no current flow through RDHEMT 100. When Vgs=0 and Vdsis less than −1.6 volts, there is a reverse current flow through RDHEMT100. This current flow at the voltage threshold of −1.6 volts is whatallows RDHEMT 100 to clamp the voltage on line 107 beginning where thevoltage on line 107 is 1.6 volts more than V+.

Because drain 112 of RDHEMT 110 is connected to line 107, line 107 isvoltage clamped from being significantly more negative than referencevoltage −V from reference voltage 106. When the voltage on line 107 isdecreased to be much less than reference voltage −V, the drain to sourcevoltage or Vds of RDHEMT 110 will decrease and go negative. As thevoltage on line 107 continues to decrease, the magnitude of the negativedrain to source voltage of RDHEMT 110 will continue to increase untilRDHEMT 110 begins to conduct current in the reverse direction from line107 to reference voltage 106, resulting in a voltage clamping effect online 107.

The operating characteristics of RDHEMT 110 are also illustrated in FIG.2 for the case where Vgs=0. When Vgs=0 and Vds is greater than −1.6volts, there is no current flow through RDHEMT 110. When Vgs=0 and Vdsis less than −1.6 volts, there is a reverse current flow through RDHEMT110. This current flow at the voltage threshold of −1.6 volts is whatallows RDHEMT 110 to clamp the voltage on line 107 beginning where thevoltage on line 107 is 1.6 volts less than reference voltage −V. ForRDHEMT 110, therefore, the voltage threshold of −1.6 volts is referredto herein as the reverse conduction onset voltage, or as the clampingvoltage. The voltage at gate 103 and the voltage at gate 113 can bevaried to modify the clamping voltage for RDHEMT 110. In general, theclamping voltage will be at the reverse conduction onset voltage.

FIG. 4 shows voltage clamping circuit 119 used for electrostaticdischarge (ESD) protection on an input pad 115 of an integrated circuit116. When voltage on input pad 115 experiences an ESD or over voltageevent, the voltage on input pad 115 can go positive or negative relativeto the Gnd (−V) or reference voltage +V. Voltage clamping circuit 119assures that the voltage does not go too far above reference voltage +Vor too far below GND. As discussed above, beginning where the voltage oninput pad 115 (and thus line 107) is 1.6 volts more than V+, there is areverse current flow through RDHEMT 100. This current flow at thereverse conduction onset voltage of −1.6 volts is what allows RDHEMT 100to clamp the voltage on input pad 115 beginning where the voltage oninput pad 115 is 1.6 volts more than V+. Likewise, beginning where thevoltage on input pad 115 is 1.6 volts less than V1 (Gnd), there is areverse current flow through RDHEMT 110. This current flow at thereverse conduction onset voltage of −1.6 volts is what allows RDHEMT 110to clamp the voltage on input pad 115 beginning where the voltage oninput pad 115 is 1.6 volts less than V−.

The foregoing discussion discloses and describes merely exemplarymethods and embodiments. As will be understood by those familiar withthe art, the disclosed subject matter may be embodied in other specificforms without departing from the spirit or characteristics thereof.Accordingly, the present disclosure is intended to be illustrative, butnot limiting, of the scope of the invention, which is set forth in thefollowing claims.

What is claimed is:
 1. A voltage clamping circuit, comprising: a firstreference voltage; a second reference voltage; a voltage clamped line; afirst reverse direction high-electron-mobility transistor, the firstreverse direction high-electron-mobility transistor including: a sourceconnected to the voltage clamped line, a gate connected to the voltageclamped line, and a drain connected to the first reference voltage; and,a second reverse direction high-electron-mobility transistor, the secondreverse direction high-electron-mobility transistor including: a sourceconnected to the second reference voltage, a gate connected to thesecond reference voltage, and a drain connected to the voltage clampedline.
 2. A voltage clamping circuit as in claim 1, wherein the firstreverse direction high-electron-mobility transistor is a Gallium nitridehigh-electron-mobility transistor where an Aluminum Gallium Nitride(AlGaN) region is formed over a non-doped narrow-bandgap channel layerof Gallium Nitride (GaN).
 3. A voltage clamping circuit as in claim 2,wherein the second reverse direction high-electron-mobility transistoris a Gallium nitride high-electron-mobility transistor where an AluminumGallium Nitride (AlGaN) region is formed over a non-doped narrow-bandgapchannel layer of Gallium Nitride (GaN).
 4. A voltage clamping circuit asin claim 1, wherein the voltage clamping circuit provides electrostaticdischarge protection to an integrated circuit.
 5. An electrostaticdischarge protection circuit, comprising: a first reference voltage; asecond reference voltage; an input pad to an integrated circuit; a firstreverse direction high-electron-mobility transistor, the first reversedirection high-electron-mobility transistor including: a sourceconnected to the input pad, a gate connected to the input pad, and adrain connected to the first reference voltage; and, a second reversedirection high-electron-mobility transistor, the second reversedirection high-electron-mobility transistor including: a sourceconnected to the second reference voltage, a gate connected to thesecond reference voltage, and a drain connected to the input pad.
 6. Anelectrostatic discharge protection circuit as in claim 5, wherein thefirst reverse direction high-electron-mobility transistor is a Galliumnitride high-electron-mobility transistor where an Aluminum GalliumNitride (AlGaN) region is formed over a non-doped narrow-bandgap channellayer of Gallium Nitride (GaN).
 7. An electrostatic discharge protectioncircuit as in claim 6, wherein the second reverse directionhigh-electron-mobility transistor is a Gallium nitridehigh-electron-mobility transistor where an Aluminum Gallium Nitride(AlGaN) region is formed over a non-doped narrow-bandgap channel layerof Gallium Nitride (GaN).
 8. An electrostatic discharge protectioncircuit as in claim 5, wherein the voltage clamping circuit provideselectrostatic discharge protection to an integrated circuit.
 9. A methodfor clamping a voltage on a voltage clamped line, comprising: providinga first reference voltage; providing a second reference voltage;connecting a source and a gate of a first reverse directionhigh-electron-mobility transistor to the voltage clamped line;connecting a drain of the first reverse direction high-electron-mobilitytransistor to the first reference voltage; connecting a source and agate of a second reverse direction high-electron-mobility transistor tothe second reference voltage; and connecting a drain of the firstreverse direction high-electron-mobility transistor to the voltageclamped line.
 10. A method as in claim 9, wherein the first reversedirection high-electron-mobility transistor is a Gallium nitridehigh-electron-mobility transistor where an Aluminum Gallium Nitride(AlGaN) region is formed over a non-doped narrow-bandgap channel layerof Gallium Nitride (GaN).
 11. A method as in claim 10, wherein thesecond reverse direction high-electron-mobility transistor is a Galliumnitride high-electron-mobility transistor where an Aluminum GalliumNitride (AlGaN) region is formed over a non-doped narrow-bandgap channellayer of Gallium Nitride (GaN).
 12. A method as in claim 9, wherein theclamped voltage provides electrostatic discharge protection to anintegrated circuit.
 13. A method as in claim 9, wherein the clampedvoltage provides electrostatic discharge protection to an input pad ofan integrated circuit.